Oct 11, 2007 - We report a novel nanoscale thermal platform compatible with extreme temperature ... time high-resolution transmission electron microscopy.
PHYSICAL REVIEW LETTERS
PRL 99, 155901 (2007)
week ending 12 OCTOBER 2007
Probing Nanoscale Solids at Thermal Extremes G. E. Begtrup,1,2 K. G. Ray,1 B. M. Kessler,1 T. D. Yuzvinsky,1,2,3 H. Garcia,1 and Alex Zettl1,2,3 1
2
Department of Physics, University of California at Berkeley, 94720 Berkeley, California, USA Materials Sciences Division, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA 3 Center of Integrated Nanomechanical Systems, 94720 Berkeley, California, USA (Received 30 April 2007; revised manuscript received 13 June 2007; published 11 October 2007)
We report a novel nanoscale thermal platform compatible with extreme temperature operation and realtime high-resolution transmission electron microscopy. Applied to multiwall carbon nanotubes, we find atomic-scale stability to 3200 K, demonstrating that carbon nanotubes are more robust than graphite or diamond. Even at these thermal extremes, nanotubes maintain 10% of their peak thermal conductivity and support electrical current densities 2 108 A=cm2 . We also apply this platform to determine the diameter dependence of the melting temperature of gold nanocrystals down to three nanometers. DOI: 10.1103/PhysRevLett.99.155901
PACS numbers: 65.80.+n, 07.20.Ka, 81.07.b
The versatility of carbon-carbon bonding underpins a wealth of extraordinary physical properties. Of the two common allotropes of carbon, sp3 -bonded diamond is electrically insulating and displays exceptional hardness and thermal conductivity, but it is metastable and spontaneously reverts to graphite at elevated temperatures [1]. sp2 -bonded graphite is electrically conducting and very stiff in the sheet direction, but it sublimes at temperatures as low as 2400 K [2]. Carbon nanotubes, which can be grown with near atomic perfection, capitalize on the extraordinary strength of the sp2 hybridized carbon-carbon bond (one of the strongest in nature), and at room temperature exhibit phenomenal electrical and thermal conductivity as well as outstanding mechanical properties. Furthermore, theoretical studies [3] indicate that nanotubes should withstand extreme temperatures, perhaps as high as 4000 K. Probing the thermal properties of nanoscale systems at very high temperature is technically challenging due to a variety of complications including the breakdown of supporting materials and calibration uncertainties. Something as seemingly straightforward as measuring local temperature becomes problematic on the nanoscale, especially at high temperature. We have developed a thermal test platform capable of operating at extreme temperatures while providing local temperature information with nanoscale resolution. We apply this platform to an investigation of the high-temperature properties of multiwall carbon nanotubes (MWNTs) and probe the limits of nanotube breakdown in vacuum and the thermal conductivity of nanotubes in the extreme high-temperature limit. In addition, we combine this new technique with calibrated MWNT heaters to address the size dependence of the melting point of metallic nanocrystals. Figure 1 shows a schematic of the thermal test platform in different stages of construction and operation. Figure 1(a) shows an electrically conducting sample mounted on a custom-fabricated thin silicon nitride (Si3 N4 ) membrane [4] that is transparent to high energy electrons, allowing real-time observation in a transmission electron microscope (TEM). As shown in Fig. 1(b), a two0031-9007=07=99(15)=155901(4)
dimensional array of single-shot nanoscale thermometers is formed by subsequently depositing metallic nanoparticles onto the sample and membrane. As electrical current is driven through the sample, Joule heating causes the temperature of the sample (and supporting membrane) to increase. The temperature of different portions of the sample is determined locally by observing the onset of local melting and evaporation of the nanoparticle thermometers. Figure 1(c) shows the sample heated with a higher bias. As the temperature continues to increase, the single-shot thermometers show a ‘‘melting front’’ receding from the sample and forming a distinctive pattern on the membrane.
FIG. 1 (color online). Design and operation of the thermal test platform. (a) A nanoscale sample is electrically contacted on a suspended membrane. (b) Nanocrystal thermometers are deposited on the sample. The sample is resistively heated via an electrical current, I, causing the nanocrystals to melt, yielding the temperature of the sample. (c) At higher bias, the melting nanocrystals yield an isothermal line which fans out across the membrane.
155901-1
© 2007 The American Physical Society
PRL 99, 155901 (2007)
PHYSICAL REVIEW LETTERS
This front corresponds to an isothermal line that, by employing finite element analysis to solve the heat distribution profile, can be used to extract the position-dependent temperature along the sample itself. As we demonstrate below, the nanoparticle thermometry method allows not only the local temperature of the sample to be determined (even when it greatly exceeds the melting point of the thermometers), but also its temperature-dependent thermal conductivity. We apply the thermal measurement platform to MWNTs. Although theoretical studies suggest that nanotubes are surprisingly stable at thermal extremes, the breakdown temperature of MWNTs in vacuum has not been directly determined [5,6] Although the temperaturedependent thermal conductivity of MWNTs is well established below room temperature [7], in the extreme high-temperature limit remains largely unexplored. Figure 2 shows a series of TEM images of a MWNT mounted on the thermal test platform, together with nanoparticle thermometers. Small gold nanoparticles (typically
